In order for an electronic device such as a PAD to be compatible with a 5V Universal Serial Bus (USB) power supply and DC conversion efficiency, the existing PAD devices are provided with a 5V adapter. However, the voltage output from a rechargeable battery of one string of cells is not constant, but may have a discharge voltage from 4.2V to 3.3V, for example. Therefore, to charge a rechargeable battery, a voltage provided by the adapter needs to be stepped down by a buck circuit before it is used to charge the rechargeable battery. Also, when the rechargeable battery is discharged to supply power to other electronic elements of the device, the voltage provided by the rechargeable battery needs to be stepped down and/or up by a buck circuit and/or a boost circuit before supplying power to the electronic elements (e.g., a 3V battery output can be stepped up to 3.3V or 5V, or stepped down to 1.8V, for power supply).
For an electronic device using a rechargeable battery, how long the battery can continuously provide power is a major concern of the user. However, during the voltage conversion process, the step up conversion results in a larger energy loss than the step down conversion. Thus, the utilization of the battery is degraded and the user requirement cannot be met.
Meanwhile, at least two converters are required for charging/power supplying of the rechargeable battery. They occupy a large space, which is undesired for miniaturization of portable devices such as PADs.